Title: Influence of synthesis parameters on iron nanoparticle size and zeta potential[1]

Authors:

Nikki Goldstein

Lauren F. Greenlee

Online Resource 1

Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) were used to obtain images of representative nanoparticle suspensions. The smaller size group of approximately 10 nm obtained in many of the dynamic light scattering measurements was observed with TEM (Figure S1).

Figure S1. Transmission electron microscopy (TEM) images of zero valent iron nanoparticles made from FeSO4*7H2O in the presence of (a) 0.8 mol ATMP per mol Fe and (b) 0.0005 mol CMC per mol Fe.

In general, the size of the particles in the electron microscopy images is typically smaller than the size measurement given by dynamic light scattering (DLS) because DLS measures the hydrodynamic diameter of the particles in solution. This measurement is based on the Brownian motion of the particles in the solvent. The hydrodynamic diameter of a particle in a specific solvent is calculated (by the DLS instrument) using the Stokes-Einstein equation and is dependent on the solvent temperature, the solvent viscosity, and the translational diffusion coefficient of the particles. To calculate hydrodynamic diameter, the Stokes-Einstein equation is given as:

where k is the Boltzmann’s constant, T is the absolute temperature, h is the solvent viscosity, and D is the translational diffusion coefficient. The hydrodynamic diameter calculated is the diameter of a sphere that has the same translational diffusion coefficient as the actual particle in solution. The hydrodynamic diameter is therefore a measurement of not only the particle but any molecules, such as organic stabilizers, that may be associated with the particle surface, as well as an associated hydration layer of water molecules. The hydration layer, or electrical double layer, is affected by the ionic strength of the solution. For non-spherical particles, the hydrodynamic diameter is a function of the particle dimension that controls the particle diffusion.

For this work, electron microscopy images indicate that the iron nanoparticles obtained were primarily spherical particles, which allows comparison between the DLS results obtained for different synthesis methods tested. However, the different stabilizers tested might have an effect on the particle size distributions obtained by DLS, due to their presence on the particle surface. Larger polymers, such as CMC, might cause a larger particle size measurement by DLS than smaller stabilizers, such as the phosphonates, because they take up more volume on the surface of the nanoparticles. The three phosphonate molecules tested are similar in size, as compared to CMC, but ATMP is half the molecular weight of HTPMP and could have less of an effect on the calculated hydrodynamic diameter than HTPMP. Furthermore, the solvent choice can also have an effect on calculated hydrodynamic diameter. The DLS instrument used was set up with the viscosity of each solvent used (i.e., water, methanol, acetonitrile, or isopropanol), but the conformation of the stabilizer on the surface of the particles, as well as the size of the electrical double layer around the nanoparticle, will change with different solvents. All of these factors tend to make the measured particle size from DLS appear larger than the size of the particles imaged with electron microscopy.

In the FESEM images, zero valent iron nanoparticles made with FeCl3 (Figure S2) appear less uniform than those made from FeSO4*7H2O (Figure S3), with a wider range of particle sizes visible. The particles shown in Figure S2a and b can be compared to the DLS results shown in Figure 2 of the main article. The FESEM image for particles formed in the presence of 0.05 DTPMP reveal two particle size groups, at approximately 50 nm and 200 nm. These two particle sizes correspond to the bimodal particle size distribution obtained in Figure 2 (labeled as DTPMP FeCl3). For the synthesis method that involved 0.05 HTPMP and FeCl3 (Figure S2b), there appear to be some larger particles and a portion of the sample where individual particles are difficult to resolve in the FESEM. This particle population may correspond to the smaller particle size of 15.5 nm ± 1.8 nm obtained using DLS. While the microscopy images and the DLS data appear to correspond for the DTPMP and HTPMP stabilizers, the results for ATMP are not as straight forward. The microscopy images for 0.05 ATMP for the two different iron salts show a more uniform particle size and particle morphology for FeSO4*7H2O (Figure S3a) than for FeCl3 (Figure S2c), whereas the DLS measurements (data not shown) resulted in virtually identical particle size distribution curves. In this case, the microscopy images reveal differences that are not detectable from the DLS data. While primarily larger particles are visible in the FESEM image of Figure S3a, TEM images similar to those shown in Figure S1 were also obtained for an ATMP:Fe ratio of 0.05. The microscopy results indicate that even though the volume particle size distribution predicts most of the particle volume will be in particles with a diameter of approximately 8 nm (main article, Figure 6a, control sample), there is a significant portion of the particle population in the larger particle size range (main article, Figure 3a). Finally, the FESEM image of nanoparticles synthesized from FeSO4*7H2O in the presence of 0.3 mol DTPMP per mol Fe appear to be aggregated together (Figure S3b), a result that supports particle size distributions obtained (Figure 4 in the main article). These examples illustrate the importance of using multiple different characterization techniques to evaluate nanoparticles after synthesis.

Figure S2. Field emission scanning electron microscopy (FESEM) images of zero valent iron nanoparticles made from FeCl3 in the presence of (a) 0.05 mol DTPMP per mol Fe, (b) 0.05 mol HTPMP per mol Fe and (c) 0.05 mol ATMP per mol Fe.

Figure S3. FESEM images of zero valent iron nanoparticles made from FeSO4*7H2O with a molar ratio of (a) 0.05 ATMP:Fe and (b) 0.3 DTPMP:Fe.

For higher molar ratios of ATMP:Fe and DTPMP:Fe, the particle size distributions indicate that the majority of the particles are quite small. FESEM images (Figure S4) suggest that the larger (~100 nm) particles may have formed from the aggregation of smaller particles, and there is evidence of smaller structures that may have been dispersed particles in the dynamic light scattering measurement. The small particles in the FESEM images of Figure S4 appear to be approximately 10 nm, and the TEM images in Figure S1 confirm this conclusion. These microscopy images appear to support the DLS results for the higher molar ratios of ATMP (main article, Figure 3b) and DTPMP (main article, Figure 4a).

Figure S4. High magnification ((a) 200,000x and (b) 500,000x) FESEM images of zero valent iron nanoparticles made from FeSO4*7H2O with a molar ratio of 0.5 ATMP:Fe.

The molecular structures of the three phosphonate stabilizers tested in this work are shown in Figure S5.

Figure S5. Molecular structures of phosphonate stabilizers: (a) Aminotris(methylenephosphonic acid) or ATMP, (b) Diethylenetriamine penta(methylene phosphonic) acid or DTPMP, and (c) Bis(hexamethylene triamine penta(methylenephosphonic acid)) or HTPMP.

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[1] Contribution of NIST, an agency of the US government; not subject to copyright in the United States.